Sidechaining, Sort Of

You want downward expansion, not compression, like the PC4K Expander/Gate. Set the bus level so that's the desired volume when the piano is loud, and the expansion ratio so that the soft parts are the desired level with respect to the piano when it's soft.
 
The Sonitus compressor can also do compression ratios below 1:1, which gives expansion. Although it's not really intended to do expansion, you may be able to wrestle the ratio, theshold, and output level to produce the desired result.

I'm a fan of this technique, too. However, I've never been happy with the results of rule-based volume tracking (e.g. this bus goes down 3 dB so that bus goes down 3 dB too) because it doesn't work equally well for all parts of a song.
 
There may be places where you want the piano's level to be much higher than everything else and other places where you want them to be more closely matched. Changing the ratio as the song progresses can add dynamic movement to the arrangement, such as gradually raising the backing instruments' levels. In a sidechain scenario that means another automation lane for the send.
 
You also have to be careful with abrupt volume changes that can cause unwanted pumping. If the piano is the featured instrument, it needs to preserve its own micro-dynamics independently. Leaving such decisions to a dumb plugin can result in a robotic-sounding mix.
 
The solution is hand-drawn automation. Tedious and time consuming, but it'll allow you to choose between sudden or gradual volume changes, whichever is appropriate for a phrase. It'll allow you to adjust the balance for best blend, e.g. a greater ratio on the verse and a lower ratio on the chorus. Or a gradually-reduced ratio as the song progresses.
 
So while it's tempting to experiment with exotic solutions, don't forget the old-school ways. They're still around for a reason.